This invention relates to a positive type photoresist composition used for preparation of a printed circuit board or an integrated circuit. More particularly, it relates to a positive type photoresist composition which, when coated or electrodeposited on a board, forms a film exhibiting no tack at room temperature and excellent adhesive properties with respect to the board.
For keeping up with recent progress in electronic appliances, higher densification and integration as well as a finer circuit pattern are required of a printed circuit board as a carrier of an electronic circuit. For this reason, a surface packaging system is applied extensively for attaching electronic components to the board, in which there are formed increasing numbers of through-holes which play the role of electrical connection across the front and back sides of the board. By the through-holes are meant apertures the inside wall of which is plated with metal and which play the role of electrical connection across the front and back sides of the board and fixture of the electronic components on the board.
For preparing the printed circuit board having these through-holes and fine circuit patterns, it is necessary to protect the inner wall of the through-holes from the etching liquid.
However, with the method of laminating a photosensitive film known as a dry film employed extensively at present, it is difficult to laminate the metal surface uniformly. Moreover, since the film itself has a larger thickness of the order of 50 .mu.m in general, and since the pattern of the etching resist produced upon exposure and development is not sharp, the through-holes with small diameters cannot be coated satisfactorily. Hence, with the laminating method, the fine circuit pattern cannot be realized.
On the other hand, with the method of uniformly applying an etching resist by electrodeposition, if a conventional negative type electrodeposition photoresist consisting mainly of a light curable resin is employed, the volume of light irradiated into the through-hole and hence the curing effect falls short, so that the electrically conductive layer of the inner wall of the through-hole can hardly be protected.
For obviating these shortcomings, researches are conducted at present for developing a positive type photoresist in which an exposed area is dissolved during development and an unexposed area functions as a protective layer, or a positive type electrodeposition photoresist capable of electrodeposition coating.
So long as the positive type photoresist is concerned, it has been proposed to use a quinone diazide compound and an alkaline solution as a photosensitive agent and as a developing solution, respectively. With the positive type photoresist, the area irradiated with light is rendered alkali-soluble so that the resist is dissolved and removed, while the area not irradiated with light is alkali-insoluble and hence is left without being removed. As a result, a protective layer may be formed without light irradiation on the inner wall of the through-hole which is hardly accessible to light.
It is noted that novolak or vinyl resins are currently employed as a resin which is a main component of the positive type photoresist employing the quinone diazide compound as the photosensitive agent. However, the novolak resin exhibits only poor adhesiveness to the board. On the other hand, the vinyl resin is unable to cause photomask adhesion because of tack in the coating film before and after irradiation, while exhibiting poor compatibility with the quinone diazide compound and poor stability of the electrodeposition liquid.